Non-destructive methods for determining material properties of solids are known in the art, and are commercially important, for example, for testing structural parts of aircraft, and other vessels, test and verification of production parts, structural pieces, et cetera. Eddy current testing is one well-known such method for determining material properties of solids that vary with magnetic and electrical properties of the material. In standard eddy current testing, a circular coil carrying an AC current is placed in close proximity to an electrically conductive specimen. The alternating current in the coil generates a changing magnetic field, which interacts with the test object and induces eddy currents therein. Variations in the phase and magnitude of these eddy currents can be monitored using a second “search” coil, or by measuring changes to the current flowing in the primary “excitation” coil. Variations in the electrical conductivity or magnetic permeability of the test object, or the presence of any flaws therein, will cause a change of eddy current flow and a corresponding change in the phase and amplitude of the measured current. This is the basis of standard eddy current inspection techniques.
It is also known to use pulsed excitation of the test coil for the detection and quantification of corrosion and cracking in multi-layer aluminum aircraft structures. Pulsed eddy current signals consist of a spectrum of frequencies, meaning that, because of the skin effect, each pulse signal contains information from a range of depths within a given test specimen. In addition, the pulse signal low-frequency components provide excellent depth penetration.
The response to a given input electric field depends on many factors, including the distance between the sample and the source of the electric field, and the distance between the sample and the detector of the response. The cumulative effect of the distances between the sample and the source and detector is referred to as lift-off
U.S. Pat. No. 6,344,741 to Giguere et al. describes a method of eddy current testing in which a transmitter coil induces a magnetic field in a test object, when excited with a square wave current (pulse). The use of a square wave function produces a time-varying magnetic field and provides for a wide range of frequency excitation. The induced eddy currents flow at specific depths within the test object and decay over a period of time after the magnetic field being generated by the coil is terminated. Various sensors can be used to capture the time-domain variation of the magnetic flux.
U.S. Pat. No. 6,344,741 uses the fact that there is a point in time at which two or three lift-off balanced responses (defined with respect to a ½ cycle of the square wave) intersect. The patent teaches selecting a representative area of the structure that has no defect, to provide (at least two, but preferably three) calibration curves of balanced response signals at different lift-off points. The lift-off point of intersection is where the curves intersect. The time of occurrence of that lift-off point of intersection will be the same for any lift-off.
Applicants have found that, unexpectedly, similar constant lift-off point calculations can be used in sinusoidal eddy current testing. In this application Lift-Off point of Intersection (LOI) is used to refer to a phenomenon of a common point of intersection of response curves independently of the lift-off spacing. The LOI time can be detected by overlaying response curves of corresponding different lift-offs, with each response curve being aligned by a fixed relative off-set with respect to an input signal. Applicants investigated whether the LOI phenomenon recurred when different input excitation functions are used, and found that sinusoidally driven eddy currents also exhibit the common LOI. Applicants have provided an explanation for the basis of the existence of the LOI in the sinusoidally driven eddy currents based on a Fourier series decomposition of the square wave form. This explanation has been demonstrated in principle by aggregating response curves of a plurality of odd harmonic sinusoidal frequencies of a base frequency of a square wave function, and comparing the aggregate response with the response to the square wave function. In all cases a LOI is detected. The LOI is detected for a wide range of lift-off values, in the range of micrometers to millimeters.